Aggregate Lines Based on Distance between Segments

Question

I have a VectorTile basemap that I am optimizing, (working on reducing the size of individual tiles), the major chunk of tile size is road data (geometry and attributes). I am working on simplifying the individual road geometries based on the Zoom Level, and aggregating the roads based on zoom levels as well.

The roads are stored in a Postgis table ( individual table for each zoom level ) and I want to aggregate the roads based on the zoom level they will be displayed at. For example, In the Zoom Level 5 table I want to aggregate the roads that are within a certain distance of one another, and make a single line for the road segments:

Since it is not possible to make out individual roads at that zoom level:

How can I aggregate the road geometries based on the distance between road segments?

PS: I am working on PostGIS and QGIS on Linux, but I am open to solutions using any Open Source platform or technology

Answer 1

You could try looking int this OS option, Skeletron, it:

generalizes collections of lines to a specific spherical mercator zoom level and pixel precision, using a polygon buffer and voronoi diagram

It is based off of a 1996 paper by Alnoor Ladak and Roberto B. Martinez, "Automated Derivation of High Accuracy Road Centrelines Thiessen Polygons Technique"

There are some dependecies required:

Required dependencies:
- qhull binary (http://www.qhull.org)
- shapely 1.2+ (http://pypi.python.org/pypi/Shapely)
- pyproj (http://code.google.com/p/pyproj)
- networkx 1.5+ (http://networkx.lanl.gov)
- StreetNames 0.1+ (https://github.com/nvkelso/map-label-style-manual/tree/master/tools/street_names)

Note: This is an older model/tool that can be applied, just as a reference for you to explore.

Answer 2

ArcGIS is the simplest if it is at all an option. I know the OP specifies OS but I mention this as the OS options just don't look very helpful compared to : http://resources.arcgis.com/en/help/main/10.1/index.html#/Merge_Divided_Roads/00700000000w000000/

Answers to this question give some Open Source options : How can I merge/collapse nearby and parallel road lines (eg. a dual-carriageway) into one single line?

If you code you could calculate the mean coordinate position on a vertex basis, if not then:

1. buffer the roads with a "merge all" option on the output.
2. Get the centre line of the polygon
3. Split the lines at junctions with JUMP Split Lines at Intersection of Other Lines
4. Spatial Join the attributes back on

There doesn't seem to be a good OS equivalent to ArcGIS 'Polygon to Centre line' tool either. I might suggest :

a. Create voronoi diagram and take the skeleton.
b. Create a Euclidean distance raster and extract the central value and convert to a line

... probably some others as well.

Finally, depending on the scale and application.. it might also be worth just deleting one of the two lines - will the imprecision matter?